You might want to get in touch with an organization called Parent 2 Parent (P2P). They are a group that has been around for many years. They have resource info on their site and regular meetings for parents to discuss issues. I’m sure it must include parents of adult children. They also have an annual conference. These are parents, not lawyers - however they will council members to contact a lawyer when needed,

The Texas P2P is very strong and has a good website of info:
https://www.txp2p.org/
National website with links to every state with a chapter:
https://www.p2pusa.org/parents/
The Helen Journal, named after Helen Keller, also has good info. Your adult child can designate a health care proxy, I believe this is a simpler solution to guardianship. See info below and web link.

“Parents should be aware of the basics of patient rights.
Their adult child is presumed to be competent to make his or her own medical decisions. Their child has a right under the ADA to have a support person help them make medical decisions. And federal law allows them to designate a parent as a patient representative who has access to medical records. Very little capacity is needed to designate a parent as a health care proxy. Many states recognize the validity of supported decision-making agreements (SDM) where a trusted supporter helps the patient make medical decisions. Providers have an obligation to show clear evidence of incapacity if they refuse to accept the patient’s authority to make his or her medical decisions or to delegate such authority. Parents have the right to assist their adult child in asking for a second opinion if a provider decides the patient lacks capacity for a particular decision. They have the right to help the patient file an appeal or a complaint for disability discrimination against the provider. “
https://helenjournal.org/september-2023/medical-decision-making-what-you-should-know
The US Dept of Health and Human Services, Children’s Bureau provides funds to states to handle guardianship. Your state probably has a website with information specific to your location.
https://www.acf.hhs.gov/cb/focus-areas/guardianship